The efficiency of gas turbine engines depends in part on maintaining dimensional tolerances during all phases of engine operation. A particularly difficult dimensional tolerance to maintain is that between the free tips of turbine rotor blades and the surrounding shroud. Typically, the rotor blades reach an elevated temperature during operation resulting in their radial growth. During periods of maximum growth the blade tips may wear down as they rub on the shroud thereby increasing the gap between blade tips and shroud during other periods of engine operation. One means for preventing the blades from contacting the shroud is to allow a sufficient gap between blade tips and shroud for maximum blade growth. If such a gap is provided, significant quantities of gas may leak therethrough during periods of non-maximum growth, thereby reducing the turbine efficiency.
An alternative solution for maintaining a good seal during blade growth is to employ an abradable shroud material. Such material is intended to wear down as rotating blade tips make contact therewith. Efforts to develop useful shrouds by this approach have experienced problems. For example, shrouds may be too soft and easily erodable or they may be susceptible to oxidation thereby being degraded. When shrouds are hardened to adequately resist the severe turbine environment or from engine operation, excessive blade wear occurs. In either case the resultant increase in blade tip clearance contributes significantly to turbine performance deterioration.
A further solution to maintaining improved seal clearance has been made possible by the availability of oxidation resistant metallic materials in foil form. One type of seal using such materials is disclosed in U.S. Pat. No. 3,916,054 issued Oct. 28, 1975 to Long et al. for COMPLIANT STRUCTURAL MEMBERS. The Long seal consists of corrogated metallic strips disposed in side by side relationship in the circumferential direction of blade rotation. This seal is typical of prior art seals in that the general orientation of the sealing strips is parallel to the direction of blade rotation. In addition, it is believed that the Long seal is abradable and permanently deforms to the rotating blade tips.